By Gary Fong (Project Manager/Manufacturing)
To begin with, it is important to understand the design process of our rover. The RoSco has four track assemblies that had to be constructed through Solidworks. This comes out to eight wheels, four tracks, 4 legs, pan and tilt, and one chassis that had to be designed. The tracks and wheels were designs that were obtained through Mike Pluma which I modified a bit to fit our requirement to overcome obstacles.
This leg is the servo-motor connection that allows the rover to stand up and move forward/backward. There is a hole in the front of the legs to allow the front wheel to freely rotate. The empty box will contain a Pololu gearmotor with the drive shaft sticking out to latch onto the drive wheel. The small notches on the box allow a “puzzle” piece to fit on with an opening for the drive shaft to stick out. The reason for this piece was to keep the motor in a sealed container to prevent any water from getting in.
Figure 2. and Figure 3. RoSco Complete Assembly back and front view
Figure 4. and Figure 5. Drive and front wheel originally 45mm
Figure 6. Tracks
One of the major issues that I had was actually calculating the dimensions needed for the design. From the following diagram, Pololu Gearmotor, I know that the length of the motor is 69.2 mm, width is 25.8 mm, and the height is 22.3 mm. The drive shaft is 7 mm in diameter, so the puzzle piece has to have an opening to accommodate for that. I made the entire leg approximately 163 mm, or 6.4 inches to allow extra room for the chassis height and still pass over the obstacle. I made the box to hold the motor 70 mm in length, 32 mm wide, and 29.1 mm high.
Figure 7. and Figure 8. Servo Horn connection different views.
The servo connection hole is 30mm from the right to allow for some stability when the servo is connected with a servo horn, which is where the line of small holes connect to. The slightly larger hole on the left side is for the wires from the motors to thread through into the chassis.
Figure 9. Puzzle piece for bracket
The tracks were probably the most difficult to dimension because the unknown factor was the elasticity of the material that Mike Pluma was printing them with. What I tried to do was create a path that simulated the length of the tracks with the wheels. The drive wheel was 50mm in diameter and the front wheels were 48mm in diameter, so what I did was measure the distance from each origin based on my leg. The drive wheel origin was the center of the motor drive shaft.
Figure 10. Track path
For clearance height, I chose 5.5 inches or 139.7 mm. This length was from the where the servo horn was attached to the end of the support bracket with the wheel attached. The distance from each origin was 133.5mm, which was then used in Solidworks > tools > dimensions > path length. From the path length, I found the circumference to be 420.95mm, which is a diameter of 114mm using the C=pi*d equation.
From there, I edited Mike Pluma’s track design to have a diameter of 114mm. This was the best I could do without knowing exactly how much the tracks would stretch. I believe that the tracks should have been slightly smaller and the wheels a bit bigger. Unfortunately, the 3D printing for this project was a bit of a hassle and we could not produce more than one actual prototype.
Concerning the chassis, Will McKinney designed it to have openings for where the servos can fit in and any openings would be covered in plexiglass to allow anyone to see the electronics inside.
The chassis actually turned out to be the best model that was printed.
Figure 11. Chassis
For the pan and tilt, I used the previous RoSco’s as a reference to go off of. I tried to keep in mind that the pan and tilt would be using mini servos, so I tried to designed it to be light enough to allow the servos to move it without any counterweight.
Figure 12. Pan and tilt assembly
Figure 13. Exploded view of the RoSco components
The most difficult part for the 3D design in Solidworks was the wheels and tracks because I could not account for unknown variables. 3D printing in general is difficult because there can be many failed prints and sometimes parts won’t fit together perfectly.
Complete files are here Solidworks Files 2015 RoSco